1. Field of the Invention
The present invention relates to a webbing retractor in a seat belt device which restricts a vehicle occupant by a webbing belt, and more particularly, to a webbing retractor which is provided with what is known as a pretensioner for forcefully taking up a predetermined amount of the webbing belt in a sudden braking state.
2. Description of the Related Art
In today's vehicles, what is known as a seat belt device is installed that restricts a vehicle occupant by an elongated band-like webbing belt. Further, among these seat belt devices, there is a type which is provided with what is known as a pretensioner for rotating a take-up shaft forcefully by a predetermined amount in the direction in which the webbing belt is taken up so that a predetermined amount of the webbing belt is forcefully taken up, thereby not only temporarily increasing the restraining force of the webbing belt but also restricting the forward movement of the occupant in the vehicle when the vehicle executes a sudden deceleration.
This type of pretensioner is provided with a cylinder that contains agents such as a gas generating agent for rapidly generating gas when it is ignited, and an igniting agent for igniting the gas generating agent. Further, a piston is contained inside the cylinder. One end of a rack bar in the longitudinal direction thereof is fixed at one end of the piston. A pinion gear which is formed at one end of a take-up shaft and coaxially therewith is located by the side of the other end of the rack bar in the longitudinal direction thereof. The rack bar is normally separated from the pinion gear, and is able to mesh with the pinion gear by moving in a direction in which it approaches the pinion gear along the longitudinal direction of the rack bar.
When detecting means such as an acceleration sensor detects a sudden deceleration state of the vehicle, the igniting agent ignites the gas generating agent so as to rapidly generate gas. The gas then causes the piston to slide inside the cylinder. When the piston slides, the rack bar moves integrally along the longitudinal direction thereof in the direction in which it approaches the pinion gear and meshes with the pinion gear, thereby rotating the pinion gear. The rotation of the pinion gear causes the take-up shaft to rotate, and the webbing belt is taken up by a length corresponding to the rotation amount of the take-up shaft.
The take-up shaft described above is rotatably supported by a bearing portion provided in a housing normally formed from a metal plate or a like. Generally, the bearing portion is in surface contact with a surface of the outer periphery of the take-up shaft along the direction of the outer periphery thereof. Thus, if the take-up shaft is rotated, for example, in a state in which the take-up shaft is made eccentric by the rack bar meshing with the pinion gear, the frictional resistance is increased because the take-up shaft rotates in a state in which it is pressed against the inner peripheral portion of the bearing portion. The force that forces the pretensioner to rotate the take-up shaft via the rack bar and the pinion gear is converted into thermal energy by this frictional resistance, thereby decreasing the portion of the force provided to actually rotate the take-up shaft. Particularly, when synthetic resin or the like, whose rigidity is lower than that of the metal material that constitutes the take-up shaft, is used for the bearing portion in an attempt to lighten the weight of the bearing portion, the above frictional resistance causes the inner peripheral portion of the bearing portion to undergo plastic deformation and the surface of the inner periphery of the bearing portion to become rough, which further increases the frictional resistance.
In this way, in a webbing retractor provided with a conventional pretensioner, the pretensioner's force for rotating the take-up shaft is decreased by the frictional resistance. Accordingly, it has been imperative to allow for such a decrease and to generate a larger amount of gas so as to compensate for it. Thus, the device has had to be large in size.